Diode-pumped visible wavelength alkali laser

ABSTRACT

In the basic Diode-Pumped Alkali Laser (DPAL) device, excitation to the n  2 P 3/2  electronic level by a single diode laser pump source leads to a population inversion between the first excited electronic  2 P 1/2  level and the ground  2 S 1/2  level, permitting the construction of efficient, high-power, compact DPAL laser oscillators in the near infrared spectral region. The present invention extends the single-step excitation DPAL to a two-step excitation, or up-conversion DPAL to produce efficient, powerful laser operation in the visible blue and near UV spectral regions (viz., in the range 460-323 nm). The present invention describes an apparatus and method that efficiently sums the energy of two, near-infrared diode pump photons in alkali vapor atoms, followed by stimulated emission to their electronic ground levels.

[0001] This is a continuation-in-part of U.S. patent application Ser.No. 10/000,508, titled “Diode-Pumped Alkali Laser” filed Oct. 23, 2001and incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to visible and near ultravioletwavelength lasers, and more specifically to diode-pumped up-conversionalkali lasers (DPALs).

[0004] 2. Description of Related Art

[0005] With the coming of the Internet and the explosive growth in datacommunications it enabled, there has been a concomitant growth in thedemand for ever-more capable visual displays in the form of electroniccinema, home theater, desktop, and mobile displays. The growth in datageneration and communications has also created an accelerating demandfor high density data storage, taking many physical forms includingoptical data storage in video DVD disks, and in optically writtenholograms in polymer coated disks. Advanced realizations ofhigh-performance displays and data storage call for the use of compact,efficient, and low-cost visible and near ultraviolet laser sources.Direct view displays based on lasers require laser sources emitting inthe red, green, and blue spectral regions. Optical data storagerecording media achieve high recording density by making small spots inthe recording medium, whose spot diameter depends inversely in thesquare of the wavelength of the laser-marking source. Thus optical datastorage devices benefit from compact, efficient shorter wavelength lasersources. DVD disks originally utilized semiconductor laser diodesoperating at 780 nm, progressed in recent times to a wavelength of 650nm, and call for laser sources that will emit at a wavelength near ˜400nm in the near future. Thus there are continuing, and growing needs forcompact, efficient, and cost-effective visible and near ultravioletlaser sources.

[0006] More specifically, there is a large and growing demand forcommercial projection displays [1] with ever-higher technicalperformance characteristics (higher resolution, higher brightness,larger screen size, more saturated color gamut, higher efficiency, andlower cost, etc.). The xenon arc lamp found in most projection displaysis technologically the weakest link in achieving displays with thedesired characteristics. The arc lamp is generally limited in brightnessbecause its output light is radiated into all spatial directions, isinefficient in producing useful visible light, produces a great amountof waste heat, and has an awkwardly short lifetime usually measured in100's of hours.

[0007] Visible red, green, and blue (RGB) laser sources offer theprospect for overcoming most of the shortcomings of incoherent arc lampsources. Lasers are comparatively much brighter than lamps, emitrelatively pure colors that enable very high gamut saturation, and canbe scaled in output power sufficient to project bright high-resolutionimages on very large screens. The most developed visible laser sourcesfor projection displays are based on diode-pumped solid state lasers, orDPSSLs, (such as Nd:YVO₄), The near infrared radiation from the DPSSL isfrequency-doubled in a nonlinear crystal, producing either red (˜640nm), green (˜532 nm) or blue (473 nm) visible light [2]. These lasersources have working efficiencies of 2-5 percent, produce output powersup to the watt range, but have proven to be many times too expensive forwide-spread use in consumer display applications.

[0008] Lower power (1-100 mW) laser-based visible sources are beingdeveloped, based on direct frequency doubling of the near-infraredradiation from a stripe laser diode in a nonlinear crystal [3]. At theselower powers it is necessary to use a guided wave structure fabricatedin the nonlinear harmonic doubler crystal, so that a sufficientinteraction length is provided for significant harmonic generation. Themost promising results regarding output power and conversion efficiencyhave been obtained using quasi-phase-matched periodically-polednonlinear materials such a lithium niobate and lithium tantalate [3]. Inthese devices, the near diffraction-limited radiation from the stripelaser diode is focused into a channel waveguide (a few microns in width)that is fabricated in a planar wafer made of the nonlinear convertercrystal. In order to achieve reasonable conversion efficiencies (10-20%or so) the fundamental wave in the waveguide must have an intensity ofat least a few hundred kW/cm². Such an intensity is high enough thatlight-induced photorefraction occurs. This phenomenon spoils thephase-matching condition for efficient harmonic generation and greatlylimits the operating lifetime of the device, especially at the higheroutput levels [4]. This problem has proven to be most difficult ingenerating shorter visible wavelengths (e.g., s blue). Also theprecision required to fabricate micron scale diode stripe lasers andcouple them efficiently into narrow width single-mode waveguides is achallenging and relatively expensive task to perform.

[0009] Thus, the market demand for relatively lower-cost, compact,efficient, high-power (0.1 to 10 watts), and long-lived (>>20,000 hours)visible (especially blue) laser sources continues unfulfilled. Thepresent invention is offered as a solution to this market need.

[0010] In addition to high performance displays, consumer demand hascontinued to grow in the past decade [5] for video DVD disks withever-higher recording densities. Conunercial video DVD disks containinga full 2 hour-long feature film have been realized with the developmentof red (˜650 nm) laser diodes. Future higher density DVD (or DVR) disks[6] are awaiting the development of a compact, efficient laser sourceemitting at a shorter wavelength (˜420-400 nm). Laser diodes producedfrom the AlGaInN compound semiconductor material system are in earlydevelopment for this application. AlGaN laser diodes emitting severaltens of milliwatts at a wavelength of ˜410 rn have been demonstrated [7]and are in early commercial evaluation. While technically adequate, thecurrent manufacturing methods of such diodes is a low-yield process,owing to the lack of a suitably lattice-matched substrate upon which toepitaxially grow these laser diode devices [8]. Thus, novel compact,efficient, low-cost laser sources in the 420400 nm spectral regioncontinue to be of commercial interest.

[0011] In addition to video DVD disk recorders and players, yet higherdata density and access rates are needed to implement massive datastorage devices for data rich computer network applications. Holographicdata storage techniques have been under intense development in the pastdecade, and new polymer recording media have been developed forcommercial and consumer products [9]. Holographic data storage deviceswill require practical, short wavelength (˜400-410 nm) lasers emittingseveral tens to up to a ˜100 milliwatts of laser power. Such lasersources are also useful as a compact fluorescence excitation source forvarious biomedical research and diagnostic applications (such as cancerdetection, DNA sequencing, and reading proteomic assays, etc.).

[0012] In light of the foregoing, needs continue for the invention anddevelopment of efficient, compact, long-lived, visible laser sourcesoperating in the ˜400-470 nm spectral range. The present inventionaddresses those needs.

SUMMARY OF THE INVENTION

[0013] It is an object of the present invention to provide anup-conversion diode-pumped alkali laser (UC-DPAL.

[0014] It is another object of the invention to provide to provide alaser cavity formed by an input mirror and an output mirror, resonant ata wavelength λ₀₃ or λ₀₄ corresponding to wavelengths of the D_(1′) orD_(2′) transitions of an alkali atomic vapor.

[0015] Another object of the invention is to provide a gain mediumwithin a resonant cavity, where the gain medium comprises a mixture ofone or more buffer gases and an alkali vapor whose D_(1′) or D_(2′)transition wavelengths match that of the resonant laser cavity.

[0016] Still another object of the invention is to provide asemiconductor diode pump laser (or laser array) emitting at a wavelengthsuitable for optically exciting a laser gain mixture of one or morebuffer gases and an alkali vapor.

[0017] Another object of the invention is to provide a semiconductordiode pump laser (or laser diode array) emitting at a wavelengthsuitable for further optically exciting alkali atoms excited by a firstpump laser, to the n ²D_(3/2) (or similar) electronic level of thealkali atom.

[0018] Another object of the invention is to provide a method forconverting the substantially-divergent, multi-spatial-mode radiation ofsemiconductor diode laser pump arrays into a near diffraction-limited,near-single-spatial-mode, coherent laser radiation at a wavelengthshorter than those of either pump.

[0019] These and other objects will be apparent to those skilled in theart based on the disclosure herein.

[0020] The use of an alkali atomic vapor element as laser active speciein a near infrared Diode-Pumped Alkali Laser (DPAL) has been disclosed[10] in U.S. Patent Application Serial No. 10/000,508, titled“Diode-Pumped Alkali Laser” filed Oct. 23, 2001, and incorporated hereinby reference. In the basic DPAL device, excitation to the n ²P_(3/2)electronic level by a single diode laser pump source leads to apopulation inversion between the first excited electronic ²P_(1/2) leveland the ground ²S_(1/2) level, permitting the construction of efficient,high-power, compact DPAL laser oscillators in the near infrared spectralregion. The present invention extends the single-step excitation DPAL toa two-step excitation, or up-conversion DPAL to produce efficient,powerful laser operation in the visible blue and near UV spectralregions (viz., in the range 460-323 nm). The present invention describesan apparatus and method that efficiently sums the energy of two,near-infrared diode pump photons in alkali vapor atoms, followed bystimulated emission to their electronic ground levels.

[0021] In the basic infrared DPAL, only the ground and first two excitedenergy levels are involved in laser action. In the UC-DPAL device,additional higher lying electronic levels are involved in generatingvisible laser emission. In the UC-DPAL device, two diode pump sourcesare utilized. The first pump, P₁, is set to the wavelength of either ofthe first resonance (so-called) D₁ or D₂ transition wavelengths (Di: n²S_(1/2)−n ²P_(1/2), or D₂: n ²S_(1/2)−n²P_(3/2)). The second pump, P₂,is set to a wavelength that equals the wavelength of a transitionbetween either the n ²P_(1/2) level or the n ²P_(3/2) level, and the n²D₃/₂ level (or possibly another ²D_(J) level, not shown). With bothpump excitation sources present, alkali atoms are successively excitedfrom the ground n ²S_(1/2) electronic level, into either the n ²P_(1/2)or n ²P_(3/2) levels, and subsequently into the n ²D_(3/2) level. In thepresence of an appropriate buffer gas mixture, the alkali atompopulations excited to the n ²P_(1/2) and n ²P_(3/2) levels come intothermal equilibrium with each other, characterized by a temperatureequal to that of the buffer gas, due to rapid collisional mixing(exchange of energy) between these levels by the buffer gas. Similarly,due to collisional mixing among the n ²D_(3/2), n+1 ²P_(1/2) and n+1²P_(3/2) levels due to presence of an appropriate buffer gas, the alkaliatom population excited by the second step pump rapidly comes to thermalequilibrium with the latter two levels, characterized by the temperatureof the buffer gas. With the appropriate excitation fluxes from the firstand second diode pump sources, a population inversion is generatedbetween the n+1 ²P_(1/2) and n+1 ²P_(3/2) levels and the ground n²S_(1/2) level. When the doubly-excited alkali/buffer-gas mixture iscontained with an appropriate laser cavity, laser action is generated ineither of the two “second series D′-transitions” of the alkali atoms:D_(1′): n+1 ²P_(1/2)−n ²S_(1/2); D_(2′): n+1 ²P_(3/2)−n ²S_(1/2).

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 shows the ground n ²S_(1/2), first two excited n²P_(1/2;3/2) electronic energy levels, and the three next-higher lyinglevels (n+1 ²P_(1/2;3/2), n ²D_(3/2)) of the alkali atoms. These levelsare involved in the up-conversion laser action of the present invention.

[0023]FIG. 2 shows the functional elements of the up-conversion,diode-pumped alkali laser (UC-DPAL).

DETAILED DESCRIPTION OF THE INVENTION

[0024] The energy level scheme for the up-conversion DPAL (or UC-DPAL)is shown in FIG. 1. In the basic infrared DPAL [10], only the ground andfirst two excited energy levels are involved in laser action. In theUC-DPAL device, additional higher lying electronic levels are involvedin generating visible laser emission. In FIG. 1, n is the principalquantum number of the alkali atoms (n=2,3,4,5,6 for lithium, sodium,potassium, rubidium, and cesium, respectively). In the UC-DPAL device,two diode pump sources are utilized. The first pump, P₁, is set to thewavelength of either of the first resonance (so-called) D₁ or D₂transition wavelengths (D₁: n ²S_(1/2)−n ²P_(1/2), or D₂: n²S_(1/2)−n²P_(3/2)). The second pump, P₂, is set to a wavelength thatequals the wavelength of a transition between either the n ²P_(1/2)level or the n ²P_(3/2) level, and the n ²D_(3/2) level (or possiblyanother ²D_(J) level, not shown). With both pump excitation sourcespresent, alkali atoms are successively excited from the ground n²S_(1/2) electronic level, into either the n ²P_(1/2) or n ²P_(3/2)levels, and subsequently into the n ²D_(3/2) level. In the presence ofan appropriate buffer gas mixture (see below), the alkali atompopulations excited to the n ²P_(1/2) and n ²P_(3/2) levels come intothermal equilibrium with each other, characterized by a temperatureequal to that of the buffer gas, due to rapid collisional mixing(exchange of energy) between these levels by the buffer gas. Similarly,due to collisional mixing among the n ²D_(3/2), n+1 ²P_(1/2) and n+1²P_(3/2) levels due to presence of an appropriate buffer gas, the alkaliatom population excited by the second step pump rapidly comes to thermalequilibrium with the latter two levels, characterized by the temperatureof the buffer gas. With the appropriate excitation fluxes from the firstand second diode pump sources, a population inversion is generatedbetween the n+1 ²P_(1/2) and n+1 ²P_(3/2) levels and the ground n²S_(1/2) level. When the doubly-excited alkali/buffer-gas mixture iscontained with an appropriate laser cavity, laser action is generated ineither of the two “second series D′-transitions” of the alkali atoms:D_(1′): n+1 ²P_(1/2)−n ²S_(1/2); D₂: n+1 ²P_(3/2)−n ²S_(1/2).

[0025] The required pump fluxes for efficient laser action in the D₁ orD₂, transitions of an alkali atom depends directly on thecollision-broadened spectral widths, effective transitioncross-sections, and the saturation fluxes of the pump and lasertransitions, which in turn depend on the type and partial pressures ofthe buffer gases utilized. The spectroscopic properties of the first andsecond series D-transitions of the alkali vapor atoms have beenextensively studied [11], first as model systems of atomic structure,and more recently as preferred species for producing Bose-Einsteincondensates. Likewise, also extensively studied have been thecollisional effects of all of the rare-gases and selected moleculargases on the spectroscopic and population kinetics of excited alkaliatoms, including spectral broadening of the D-line transitions [11-15],collisional mixing rates of excited ²P_(1/2,3/2) alkali atoms [16-23],and inelastic quenching rates of excited alkali atoms [24]. TABLE 1UC-DPAL relevant electronic level energies and transition wavelengths. nAlkali E₁ (cm⁻¹) E₂ (cm⁻¹) E₃ (cm⁻¹) E₄ (cm⁻¹) E₅ (cm⁻¹) ΔR₅₄ (cm⁻¹) 6Cs 11,178.2 11,732.4 21,765.7 21,946.7 22,588.9 642.2 5 Rb 12,578.912,816.4 23,715.2 23,792.7 25,700.6 1,907.6 4 K 12,985.2 13,042.924,701.4 24,720.2 30,185.6 5,465.4 3 Na 16,956.2 16,973.4 30,266.930,272.5 34,548.8 4,276.3 2 Li 14,903.7 14,904.0 30,925.3 30,925.431,283.1 357.7 n Alkali λ_(pump,01) (nm) λ_(pump,02) (nm) λ_(pump,15)(nm) λ_(pump,25) (nm) λ_(laser,03) (nm) λ_(laser,04) (nm) 6 Cs 894.6852.3 876.4 921.1 459.4 455.6 5 Rb 795.0 780.3 762.1 776.1 421.7 420.3 4K 770.1 877.7 581.4 583.3 404.8 404.5 3 Na 589.8 589.2 568.4 569.0 330.4330.3 2 Li 671.0 671.0 610.5 610.5 323.4 323.4

[0026] Table 1 gives a summary of the electronic level energies andcorresponding transition wavelengths relevant to UC DPAL devices, foreach of the five alkali atoms. From Table 1, it is noted (in italics)that the demand pump wavelengths for the cesium and rubidium UC-DPALslie in the 762-921 nm spectral range, for which powerful and efficienthigh power laser diode and diode arrays are commercially available.Therefore, these particular alkali atoms are preferred alkali atoms forpractical UC-DPAL devices.

[0027] The basic functional elements of an UC-DPAL device are shown inFIG. 2. The UC-DPAL laser gain cell 8 contains the laser active alkalivapor and an appropriate buffer gas (e.g., a mixture of a rare gas suchas helium and a selected molecular gas a such as ethane). Generally, thegain cell 8 will have a length “L” and transverse cross-section that isgenerally circularly symmetric with radius “r”, and having an aspectratio, L/r, of typically >10. However, the cross-sectional shape of thegain cell may take many forms (circular, square, rectangular, or higherpolygonal form) and may also be designated as a tube, capillary,hollow-waveguide, etc. The gain cell is fitted with flat optical windows9 and 10 at either end so as to contain the alkali atomic vapor. Thecell windows 9 and 10 are coated on their exterior surfaces with amultilayer dielectric stack to form an anti-reflection coating at bothof the pumping wavelengths, and at the operating laser wavelength(either put lambda here l₀₃ or l₀₄) of the UC-DPAL. The diode pumpsources 1 and 2 are collimated with lenses 3 and 4, respectively,spatially combined by the thin-film polarizer or dichroic beam combineroptic 11, and focused into the laser gain cell by lens 5, through lasercavity mirror 6. The laser cavity mirror 6 is coated with a dielectricstack that produces high reflectivity at the UC-DPAL laser wavelength,and high transmission in the near infrared wavelengths of the two pumps.The laser cavity output mirror 7 is coated with a dielectric stack thathighly reflects in the near infrared at the two pump wavelengths, andpartially transmits at the UC-DPAL laser wavelength (with a reflectivitythat optimizes the conversion efficiency of diode pump light to bluelaser output).

[0028] The main purpose of the buffer gas is two-fold: 1) the buffer gascollisionally broadens the optical transitions, renders the transitionsspectrally homogeneous with predominantly Lorentzian lineshapes, andfacilitates increased spectral coupling between the pump radiation andalkali atom absorption; and 2), the buffer gas collisionally relaxesdoubly-excited alkali atoms from the ²D_(3/2) level to the n+1 ²P_(1/2)and ²P_(3/2) levels (the upper laser levels for the two UC-DPAL lasertransitions, n+1 ²P_(3/2)−n ²S_(1/2) or the n+1 ²P_(1/2) n ²S_(1/2). Thebuffer gas composition and density is chosen so that this relaxationrate substantially exceeds the radiative relaxation rate of the ²D_(3/2)level.

[0029] For example, using atomic collision data from the literature[11-24], a suitable buffer gas mixture for a cesium UC-DPAL is ˜1-2 atmof helium and ˜0.1 atm of ethane. Such a buffer mixture can sufficientlyrelax the excited populations to the desired levels before significantundesired radiative emission takes place.

[0030] Up-conversion laser action can be efficient in the UC-DPALbecause the ground level population can be readily bleached usingcommercially available laser diodes, and substantial populationinversions (and small signal gain) can be produced on the n+1 ²P_(3/2)−n²S_(1/2) and the n+1 ²P_(1/2)−n ²S_(1/2) transitions. To gain insightinto the laser performance of UC-DPAL devices, it is necessary toconstruct a computer code capable of tracking alkali level populationsin all of the relevant energy levels (those strongly coupled together bythe radiation fields of the two spatially-overlapping pump beams, and bythe simultaneous oscillation and saturation of level populations due tolaser action within the laser cavity [25]). It is because the magnitudesof the collisionally-broadened alkali atom pump transition peakcross-sections are large (>10⁻¹³ cm²), and that the correspondingpopulation saturation fluxes are small (˜30 watts/cm², relative to pumpfluxes available with pump laser diodes) that substantial and practicalamounts of pump radiation absorption can be achieved within theLorentzian wings of the transitions, thus enabling practical UC-DPALdesigns.

REFERENCES

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[0056] All of the references cited herein are incorporated herein byreference.

[0057] The foregoing description of the invention has been presented forpurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. The embodiments were chosen and described to best explain theprinciples of the invention and its practical application to therebyenable others skilled in the art to best use the invention in variousembodiments and with various modifications suited to the particular usecontemplated. The scope of the invention is to be defined by thefollowing claims.

I claim:
 1. An up-conversion diode-pumped alkali laser (UC-DPAL),comprising: a laser cavity formed by an input mirror and an outputmirror, resonant at a wavelength λ₀₃ or λ₀₄ corresponding to wavelengthsof the D_(1′) or D_(2′) transitions of an alkali atomic vapor; a gainmedium within said resonant cavity, said gain medium comprising amixture of one or more buffer gases and an alkali vapor whose D_(1′) orD_(2′) transition wavelengths match that of the said resonant lasercavity; and a first semiconductor diode pump laser (or laser array)emitting at a wavelength suitable for optically exciting said laser gainmixture at a wavelength of the first series D₁ or D₂ transitions of saidalkali vapor; and a second semiconductor diode pump laser (or laserdiode array) emitting at a wavelength suitable for further opticallyexciting alkali atoms excited by the first pump laser, to the n ²D_(3/2)(or similar) electronic level of the alkali atom, the two said pumpstogether producing a population inversion and laser emission at thewavelength of the second series D_(1′) 0 or D_(2′) transitions of saidalkali vapor.
 2. The alkali vapor laser set forth in claim 1, whereinthe alkali vapor is selected from the group consisting of cesium (Cs),rubidium (Rb), potassium (K), sodium (Na) and lithium (Li).
 3. Thealkali vapor laser set forth in claim 1, wherein the buffer gases areselected from the group consisting of 1) the subgroup of rare gases(xenon, krypton, argon, neon, and helium) and/or 2) the subgroup oflight molecular gases (hydrogen, methane, ethane, propane; and theirdeuterated analogues).
 4. The alkali vapor laser of claim 1, wherein thesaid output mirror of said resonant cavity is partially transmitting ata wavelength λ₀₃ or λ₀₄ matching the wavelengths of the second seriesD_(1′) or D_(2′) transitions of the said alkali vapor, permittingout-coupling of laser radiation generated within said optically pumpedalkali vapor laser at a wavelength of λ₀₃ or λ₀₄ wherein said outputmirror is made substantially highly reflecting at a wavelength matchingthe wavelengths of the two said pump laser diodes.
 5. The alkali vaporlaser of claim 1, wherein said input mirror of said laser cavity is adichroic mirror, substantially transmitting radiation at a wavelengthsof said pump laser diodes or diode arrays, and substantially reflectingat a wavelength λ₀₃ or λ₀₄ matching the wavelengths of the second seriesD_(1′) or D_(2′) transitions of the said alkali vapor.
 6. The alkalivapor laser of claim 1, wherein a thin-film polarizer plate opticalelement is employed to spatially overlap and combine the polarizedradiation from the two said pump laser diodes, prior to directing theradiation of the combined pump beams into said gain medium cell.
 7. Thealkali vapor laser of claim 1, wherein the said alkali vapor is cesiumand the buffer gases are helium and ethane.
 8. The alkali vapor laser ofclaim 7, wherein the first semiconductor pump diode laser of claim 1emits at a wavelength of either ˜852 nm or 895 nm, matching thewavelength of the first-series cesium D₂ and D₁ transition wavelengths,respectively, and wherein the second semiconductor pump diode laser ofclaim 1 emits at a wavelength of either ˜921 nm or 876 nm, whose laseractive material is selected from the AlGaAs or InGaAsP semiconductorcompound material systems
 9. The alkali vapor laser system of claim 7,wherein the said laser cavity of claim 1 is resonant at a wavelength ofeither ˜455 nm or ˜459 nm, matching the wavelengths of the second seriescesium D_(2′) and D_(1′) transitions, respectively.
 10. The alkali vaporlaser of claim 1, wherein the alkali vapor is rubidium and the buffergases are helium and ethane.
 11. The alkali vapor laser of claim 10,wherein the first semiconductor pump diode laser of claim 1 emits at awavelength of either ˜780 nm or ˜795 nm, matching the wavelength of thefirst series rubidium D₂ and D₁ transition wavelengths, respectively,and wherein the second semiconductor pump diode laser of claim 1 emitsat a wavelength of either ˜776 nm or 761 nm, whose laser active materialis selected from the AlGaAs, AlGaAlP, or InGaAsP semiconductor compoundmaterial systems.
 12. The alkali vapor laser system of claim 10, whereinthe said laser cavity of claim 1 is resonant at a wavelength of ˜422 nmor 420 nm, matching the wavelengths of the second series rubidium D_(2′)and D_(1′) transitions, respectively.
 13. A method for converting thesubstantially-divergent, multi-spatial-mode radiation of semiconductordiode laser pump arrays into a near diffraction-limited,near-single-spatial-mode, coherent laser radiation at a wavelengthshorter than those of either pump, comprising the steps of: depositingthe radiation from said pump diodes in an alkali/buffer-gas gain mixturethrough a two step sequential absorption process, in a certain definedspatial volume (within the gain mixture cell placed in a laser cavity),and spatially over-lapping said volume with the fundamental mode of thealkali vapor laser cavity, designed to possess substantially higherlosses for higher order spatial modes than for the fundamental mode; andextracting laser output power at the shorter wavelength in thefundamental spatial mode of the laser cavity by providing the properamount of transmission of radiation at the output wavelength λ₀₃ or λ₀₄matching the wavelengths of the D_(2′) and D_(1′) transitions of saidalkali vapor.
 14. A method for producing an up-conversion diode-pumpedalkali laser (UC-DPAL), comprising: forming a laser cavity with an inputmirror and an output mirror, resonant at a wavelength λ₀₃ or λ₀₄corresponding to wavelengths of the D_(1′) or D_(2′) transitions of analkali atomic vapor; providing a gain medium within said resonantcavity, said gain medium comprising a mixture of one or more buffergases and an alkali vapor whose D_(1′) or D_(2′) transition wavelengthsmatch that of the said resonant laser cavity; providing a firstsemiconductor diode pump laser (or laser array) that can emit at awavelength suitable for optically exciting said laser gain mixture at awavelength of the first series D₁ or D₂ transitions of said alkalivapor; and providing a second semiconductor diode pump laser (or laserdiode array) that can emit at a wavelength suitable for furtheroptically exciting alkali atoms excited by the first pump laser, to then ²D_(3/2) (or similar) electronic level of the alkali atom, the twosaid pumps together producing a population inversion and laser emissionat the wavelength of the second series D_(1′) or D_(2′) transitions ofsaid alkali vapor.
 15. The method of claim 14, wherein the alkali vaporis selected from the group consisting of cesium (Cs), rubidium (Rb),potassium (K), sodium (Na) and lithium (Li).
 16. The method of claim 14,wherein the buffer gases are selected from the group consisting of 1)the subgroup of rare gases (xenon, krypton, argon, neon, and helium)and/or 2) the subgroup of light molecular gases (hydrogen, methane,ethane, propane; and their deuterated analogues).
 17. The method ofclaim 14, wherein the said output mirror of said resonant cavity ispartially transmitting at a wavelength λ₀₃ or λ₀₄ matching thewavelengths of the second series D_(1′) or D_(2′) transitions of thesaid alkali vapor, permitting out-coupling of laser radiation generatedwithin said optically pumped alkali vapor laser at a wavelength of λ₀₃or λ₀₄, wherein said output mirror is made substantially highlyreflecting at a wavelength matching the wavelengths of the two said pumplaser diodes.
 18. The method of claim 14, wherein said input mirror ofsaid laser cavity is a dichroic mirror, substantially transmittingradiation at a wavelengths of said pump laser diodes or diode arrays,and substantially reflecting at a wavelength λ₀₃ or λ₀₄ matching thewavelengths of the second series D_(1′) or D_(2′) transitions of thesaid alkali vapor.
 19. The method of claim 14, wherein a thin-filmpolarizer plate optical element is employed to spatially overlap andcombine the polarized radiation from the two said pump laser diodes,prior to directing the radiation of the combined pump beams into saidgain medium cell.
 20. The method of claim 14, wherein the said alkalivapor is cesium and the buffer gases are helium and ethane.
 21. Themethod of claim 20, wherein the first semiconductor pump diode laser ofclaim 1 emits at a wavelength of either ˜852 nm or 895 nm, matching thewavelength of the first-series cesium D₂ and D₁ transition wavelengths,respectively, and wherein the second semiconductor pump diode laser ofclaim 1 emits at a wavelength of either ˜921 nm or 876 nm, whose laseractive material is selected from the AlGaAs or InGaAsP semiconductorcompound material systems
 22. The method of claim 20, wherein the saidlaser cavity is resonant at a wavelength of either ˜455 nm or ˜459 nm,matching the wavelengths of the second series cesium D_(2′) and D_(1′)transitions, respectively.
 23. The method of claim 14, wherein thealkali vapor is rubidium and the buffer gases are helium and ethane. 24.The method of claim 23, wherein the first semiconductor pump diode laserof claim 1 emits at a wavelength of either ˜780 nm or ˜795 nm, matchingthe wavelength of the first series rubidium D₂ and D₁ transitionwavelengths, respectively, and wherein the second semiconductor pumpdiode laser of claim 1 emits at a wavelength of either ˜776 nm or 761mm, whose laser active material is selected from the AlGaAs, AlGaAlP, orInGaAsP semiconductor compound material systems.
 25. The method of claim23, wherein the said laser cavity of claim 1 is resonant at a wavelengthof ˜422 nm or 420 nm, matching the wavelengths of the second seriesrubidium D_(2′) and D_(1′) transitions, respectively.